Here is a term paper on ‘Amino Acid Metabolism’. Find paragraphs, long and short term papers on ‘Amino Acid Metabolism’ especially written for school and college students.

From nutritional aspects, the amino acids may be classified into three groups, namely dispensable, indispensable (essential) and semi dispensable amino acids. Dispensable amino acids are those which can be synthesized at the desired rates in the human body out of the substances ordinarily available in the body. Indispensable amino acids are those which cannot be synthesized in the body under ordinary circumstances.

Some of the indispensable amino acids may be synthesized under certain special circumstances when special ingredients are made available to the body but this does not happen ordinarily. The semi dispensable amino acids are those which under ordinary circumstances are synthesized in only very small quantities which do not commensurate with the body needs.

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Examples of each category are given below:

Amino Acids

For all practical purposes, it may be assumed that the indispensable amino acids are not synthesized in the body, however, these are absolutely essential for adequate growth, and hence are also known as essential amino acids. These must be obtained by the body from diet in the preformed stage. The nutritive value of a particular protein depends upon the number and quantity of essential amino acids it contains.

If a given protein is deficient even in a single essential amino acid, its ability to synthesize body proteins is considerably reduced. Such proteins are merely used by the body as source of energy. For positive nitrogen balance, it is absolutely necessary that all the essential amino acids should be obtained by the body in adequate amounts at the same time.

It is not necessary that a single protein should contain all the essential amino acids. These may be obtained from a mixture of several proteins, derived from different sources. Since amino acids are not stored as such in the body, administration of the missing essential amino acids afterwards does not serve the purpose.

Term Paper # 1. Catabolism of the Ketogenic Amino Acids:

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i. Leucine:

Leucine is converted to 3-hydroxy-3-methylglutaryl CoA (HMGCoA) by a series of reactions. The 2-ketoisocaproate dehydrogenase complex is analogous to the pyruvate and keto-glutarate dehydrogenase complexes in that thiamine pyrophosphate, lipoic acid, CoA, flavine adenine dinucleotide (FAD), and NAD+ are all involved. This complex is defective in individuals with maple syrup urine disease. In this disease the urine takes on a characteristic maple syrup smell as result of accumulated keto acids.

HMGCoA, the last product in the leucine catabolic pathway, is a precursor of cholesterol, so one might think that leucine could be used for the synthesis of steroids. Actually, leucine catabolism occurs in the mitochondria, whereas sterol synthesis is cytoplasmic; consequently, leucine produces acetoacetate and acetyl CoA exclusively. Acetoacetate is a ketone body, but any amino acid that can form acetyl CoA may be considered ketogenic, since the acetoacetyl CoA thiolase reaction is somewhat reversible.

ii. Lysine:

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Lysine is one of the few amino acids that are not delaminated in its initial catabolic reaction. Two pathways exist for its metabolism in animals; both share a modification of the α-amino group before deamination. Some still question the relative importance of the pathways. Both routes ultimately yield 2-aminoadipate. The oxidative decarboxylation of this substance is catalyzed by an enzyme complex similar to the pyruvate and ketoglutarate dehydrogenase complexes.

Further decarboxylation and oxidation yield crotonyl CoA, which is also an intermediate in fatty acid oxidation. This intermediate can be split to give acetoacetyl CoA and eventually acetyl CoA or acetoacetate. The pathway through saccharopine is probably important in humans, since patients with hyperlysinemia also accumulate saccharopine.

Term Paper # 2. Catabolism of Amino Acids that are Both Ketogenic and Glycogenic:

i. Isoleucine:

Isoleucine is catabolized by a series of reactions similar to those for leucine. The reactions before the formation of the unsaturated acid are entirely analogous to those for leucine. The enzymes are similar for each pathway, but only the 2-ketoisocaproate dehydrogenases are identical. In isoleucine catabolism the formation of acetyl CoA results from the action of a thiolase. The propionyl CoA product can be carboxylated to form methylmalonyl CoA. Methylmalonyl CoA can be isomerized to succinyl CoA, which enters the Krebs cycle and ultimately yields pyruvate, glucose, or glycogen.

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ii. Phenylalanine and Tyrosine:

Phenylalanine is converted to tyrosine by the enzyme phenylalanine hydroxylase – 

After this reaction the catabolism of phenylalanine is the same as that of tyrosine.

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The enzyme tyrosine aminotransferase (tyrosine transaminase) mediates the deamination of tyrosine. As with many other aminotransferases, α-ketoglutarate (αKG) is the amino acceptor. The synthesis of tyrosine aminotransferase can be induced in animal liver by glucocorticoids, which are steroid hormones that stimulate protein catabolism and thus increase the concentration of blood glucose.

Catabolism of Leucine

This induction leads to an increase in the messenger ribonucleic acid (mRNA) for the enzyme. The keto acid product of tyrosine transaminase action, p-hydroxyphenylpyruvate, is subsequently hydroxylated, decarboxylated, and its side chain rearranged. The product of this reaction is homogentisate, a hydroquinone. Another oxidation, catalyzed by homogentisate oxidase, opens the phenyl ring. This reaction gives rise to maleoylacetoacetate and ultimately acetoacetate and fumarate.

iii. Tryptophan:

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Tryptophan has a complex metabolism that can give rise to both alanine, which is glycogenic, and acetyl CoA, which is ketogenic. The initial reaction is an oxidation catalyzed by the enzyme tryptophan pyrrolase (tryptophan 2, 3-dioxygenase). As with tyrosine aminotransferase, the synthesis of this enzyme is induced by the administration of glucocorticoids.

Dietary tryptophan also increases the concentration of the enzyme in liver, which is the result of a decrease in the rate of enzyme degradation. In the reactions that follow, the tryptophan side-chain is cleaved to give alanine and 3-hydroxyanthranilate. The latter compound is decarboxylated and reduced to α-ketoadipate. The subsequent reactions that yield acetoacetyl CoA from α-ketoadipate are the same as those shown for lysine.

Catabolism of Lystine

Tryptophan Metabolism in Disease:

In carcinoid syndrome, a malignancy of the enterochromaffin or argentaffin cells that produce serotonin, excessive amounts of the tryptophan metabolites involved in the synthesis of serotonin are excreted, as well as serotonin itself. The biogenic amine serotonin (5-hydroxytryptamine) is synthesized from tryptophan.

Term Paper # 3. Catabolism of the Glycogenic Amino Acids:

Alanine, aspartate, and glutamate can be metabolized via Krebs cycle enzymes by reversible transamination reactions. The α-keto acids resulting from these transamination’s are pyruvate, oxaloacetate, and α-ketoglutarate, respectively. All these substances are glycogenic.

i. Glutamine and Asparagine:

Glutamine and asparagine can be converted to glutamate and aspartate by hydrolytic reactions that yield ammonia and the two glycogenic amino acids, glutamate and aspartate. The diagram summarizes the conversions of these amino acids.

The ammonia produced by the kidney in response to metabolic acidosis originates from glutamine and is released by the enzyme glutaminase. However, the amide nitrogen of glutamine is not always released as an ammonium ion. The nitrogen can be transferred enzymatically to a variety of acceptors; for example, the formation of amino sugars and the synthesis of the purine and pyrimidine rings require transfers of amide nitrogen from glutamine.

ii. Glycine, Serine and Cysteine:

Glycine, serine, and cysteine can all be metabolized to pyruvate. The enzyme serine dehydratase causes the direct conversion of serine to pyruvate. Serine can be synthesized from glycine by the transfer of a hydroxymethyl group from 5, 10-methylene THF (tetrahydrofolate). The sulfur atom of cysteine is first oxidized, the product transaminated, and the sulfite residue hydrolyzed away, leaving pyruvate.

iii. Threonine:

Threonine dehydratase, an enzyme similar to serine dehydratase, produces α-ketobutyrate. The enzyme is also induced by glucocorticoids. Subsequently, ketobutyrate is oxidatively decarboxylated to yield propionyl CoA by an enzyme complex similar to the pyruvate dehydrogenase complex. The propionyl CoA formed is then carboxylated to methylmalonyl CoA, which in turn is isomerized to succinyl CoA. Succinyl CoA enters the Krebs cycle and gives rise to pyruvate.

Catabolism of Isoleucine

iv. Methionine:

Methionine is another amino acid that produces α-ketobutyrate, and thus it has the same fate as threonine. Methyl groups derived from methionine can be transferred to various acceptor molecules, such as RNA and deoxyribonucleic acid (DNA).

Catabolism of Tyrosine

Catabolism of Tryptophan

More importantly, however, the transfer of the methionine methyl groups leaves homocysteine as the other product of the reaction.

Homocysteine has the structure:

The sulfur atom of homocysteine is transferred to serine to yield cysteine and homoserine. The carbon atoms of homoserine are the ones derived from methionine, through homocysteine. Homoserine is then deaminated and dehydrated in much the same way as threonine to yield α-ketobutyrate and eventually propionyl CoA and succinyl CoA, substances that are glycogenic.

v. Arginine:

Arginine is converted to glutamate-g-semialdehyde according to the sequence:

Note that arginase of the urea cycle participates. Glutamate semi-aldehyde is then oxidized to glutamate, and the glutamate is metabolized.

Catabolism of Theronine

vi. Proline:

Proline catabolism differs from that of most of the amino acids in that the molecule undergoes two oxidations without being deaminated. The first oxidation is mediated by a mitochondrial flavoprotein (FAD) to form an unsaturated intermediate; further oxidation using NAD+ opens the ring to yield glutamate.

vii. Valine:

As with other branched-chain amino acids, valine is first transaminated and then oxidatively decarboxylated. In this case the final product is methylmalonyl CoA, which can be isomerized to succinyl CoA. Unlike the other branched-chain amino acids, which are at least partly ketogenic, the catabolism of valine is purely glycogenic.

One sees that the branched-chain amino acids leucine and isoleucine undergo similar oxidative decarboxylations after an initial transamination to the corresponding keto acids. The enzyme complexes that catalyze these reactions resemble pyruvate dehydrogenase in that they utilize the same five coenzymes.

The aketoisocaproate dehydrogenase functions with keto acids derived from either leucine or isoleucine, and a defect in this complex causes maple syrup urine disease. Although the dehydrogenase of the valine pathway is not genetically affected in the disease, keto acids derived from valine are found in the urine; the accumulated keto acids from leucine and isoleucine can inhibit the ketovaline dehydrogenase.

viii. Histidine:

The enzyme histidase deaminates the amino acid to produce the unsaturated intermediate urocanate. Hydration causes the opening of the imidazole ring to give formiminoglutamate. Perhaps the most distinctive reaction in the pathway is the subsequent transfer of the formimino group to THF, producing glutamate and 5-formimino THF.